A. Field of the Invention
This invention relates to the field of art of level indicator control systems using capacitive probes for indication and control of a substance in a tank.
B. Background Art
It is well known that the level of a substance, i.e. a fluid or granular solid, in an open or closed tank or vessel can be measured and controlled by many, fundamentally similar, methods. Measurement and control is usually based on the concept that the change in fluid level in the tank is equivalent to displacing the top surface of the fluid.
In an earlier method of measurement and control, floats were used to detect and regulate the fluid level in a container. The method employs direct-actuated types of liquid level detectors and is applicable to open tanks or vessels which are subject to atmospheric pressure. However, when using closed tanks, water level is detected in a system under pressure. An arrangement used for this purpose includes one valve positioned at the lowest fluid level point in the tank. Periodic opening of these valves will establish the presence of either steam or water at each valve permitting an inference to be drawn concerning the actual water level in the tank.
Later detectors measured the fluid level in a tank by sensing the hydrostatic head of the fluid and converting this pressure measurement to actual fluid level height or fluid volume. For open tanks, a pressure-gauge-type instrument may be used. A connection is made to the pressure gauge at the minimum or zero fluid level. The full scale range of the gauge is made equal to the head of the fluid in the tank. There are numerous variations of this method including adaptations to tanks where the pressure gauge cannot be located at the zero level and where the medium to be measured is a solid.
A pressure gauge is not practical for measuring fluid level in a pressurized tank, since the actual level to be measured represents only a very small equivalent percentage of the static pressure of the fluid in the tank. Also, an added difficulty is that unless the tank pressure is held constant, the pressure gauge reading is of no value since the change in pressure alters the initial zero level reading. To overcome these problems, differential pressure measuring devices were used to measure the fluid level in pressurized tanks. Connections are made at both high and low fluid levels, one to each side of the differential pressure device, i.e. a Bellows-type meter. The separate connection to each side of the differential pressure device provides for a balancing of the effect of static pressure since it exerts the same force on both the high-pressure and low-pressure side of the tank. Therefore, the pressure head which actuates the detector is the difference between the constant reference and variable fluid level in the tank.
Other devices, improvements and adaptations of float-operated level sensors were developed based on the principles previously discussed. For example, a float may be connected to an electrical switch for providing an alarm, operating a solenoid valve or indicating when a discrete amount of fluid has been poured in or removed from the tank. Floats may be used to operate control valves directly to prevent further fluid flow to the tank, and displacement-type float units may be used to operate control units and remote transmitters.
Another method for detecting fluid levels in tanks utilizes the concept that certain fluids will conduct electricity, while air in a relative sense does not, so that the fluid level may be established through the physical contact of the probe and the conductive fluid. Since the change in fluid level is equivalent to displacing the top surface of the fluid, the usually linear displacement may be measured by resistive, capacitive, magnetic, or photoelectric transducers. Further methods of level detection include temperature-sensing transducers, multi-turn potentiometers operated by a float actuated cable and ultrasonic and gamma-ray adsorption.
A computer-based control system uses well known signal acquisition input instrumentation to obtain analog signals from sensors and transducers, such as capacitance probes, in the tank and transmits them to the computer. To close the fluid level control loop, D/A converters and digital output channels may be used to transmit the signals used to drive on-off fluid level controllers and actuators. Devices such as relays or stepper motors for opening and closing pneumatic fluid valves are also provided control signals from the computer along digital output channels for controlling the fluid flow into and out of the tank. The processor may, for example, compare the input signals from the fluid level transducers with upper and lower set point limits in order to control, in on-off, proportional, integral or differential modes, the fluid flow to the tank to maintain the desired liquid level within a predetermined range. Alarm monitoring and faulty transducer detection can also be performed by the computer.
Analog controllers may be used without a computer processor for controlling the level of fluid in a tank. The analog controller may either use its own set point reference voltage to control fluid input to the tank or it may accept fluid level set point limits from a central processor for the same purpose.
Output devices such as strip chart recorders using properly scaled paper, calibrated meters with d'Arsonval movement and digital displays have all been used to show the amount and the height of fluid in tanks.